It is well known that coal is the most abundant energy source in the United States. Many attempts have been made to extend the use of coal as an industrial energy source in the United States by forming coal-water and coal-oil slurries, or mixtures.
The rationale behind the formation of such slurries are numerous. First, slurries are more easily and safely transported than dry coal, the later being known to form hazardous coal dust. Additionally, these slurries are more easily stored and less subject to the possibility of explosion by spontaneous ignition. Further, the fluid nature of the slurries enables burning in existing combustion equipment that has previously been designed for the burning of fuel oil. This last advantage is significant from a cost standpoint since substantially less retrofit of combustion equipment is necessary to change from the burning of fuel oil to the burning of coal slurries, than to rework the equipment to burn dry coal.
Significant problems have been encountered in dispersing coal in water to form coal slurries, or mixtures, suitable for use as a combustion source. First, it is well known that coal slurries do not have stability toward sedimentation, that is, they tend to settle when they are stored.
Numerous methods have been utilized to prevent settling or stabilize the coal slurry. For example, it is well know that more finely ground coal is easier to disperse in water and once a slurry is formed the coal tends to stay suspended for longer periods of time. Unfortunately, the extensive grinding requires a large amount of mechanical energy which significantly increases the cost of coal water slurries, making them less economically attractive.
It is also apparent that the usefulness of the coal aqueous slurry as a combustion source depends on the amount of coal suspended or dispersed in the water. Water may be a significant factor in the cost of transportation and processing operations hence, the less water transported the greater the volume of coal that can be utilized per unit volume of coal aqueous slurry. In addition, the greater percentage of water, the more difficulty one has in burning the resulting mixture.
Many attempts have been made to form stable dispersions of coal and water through the use of additives. For example, in U.S. Pat. No. 4,242,098 issued to Braun al, it was found that small amounts of certain water-soluble polymers, when added to aqueous coal slurries, permitted the transportation of coal slurries with much higher weight percent solids content than was heretofore possible. The water soluble polymers discribed therein included poly (ethylene oxide), particularly hydrolyzed poly (acrylamide), hydroxyethyl cellulose, among others. Unfortunately, a significant amount of these additives must be utilized to provide a slurry with an exceptable shear stability and viscosity, thus increasing the cost of the resulting coal water slurry.
A more recent U.S. patent to Mark, namely U.S. Pat. No. 4,358,293, utilizes polyalkyleneoxide nonionic surfactants for forming coal-aqueous mixtures having high coal solids concentrations. This reference teaches that polyalkyleneoxide nonionic surfactants of high molecular weight having a hydrophobic portion and a hydrophilic portion enable the dispersing of coal in water to the extent of having coal solid concentrations of about 70 percent by weight or higher when the hydrophilic portion is comprised of at least about 100 ethylene oxide repeating units.
As recognized by Mark in U.S. Pat. No. 4,358,293, it is desireable to provide coal in aqueous mixture form, when only a small amount of additive materials is needed to disperse the coal to high solid concentrations. Attempts to minimize the amounts of additive utilized are directly associated with the overall cost of providing coal aqueous slurries on a commercial basis.
The present invention is directed to the utilization of surfactants such as those used by Mark in a manner enabling the use of less surfactants to stabilize coal aqueous slurries having a coal content of 70 percent or better, and further to provide coal aqueous slurries having lower viscosity and higher shear stability. Lower viscosity relates to less transportation costs by way of pumping, and greater shear stability relates to the resistance of the slurry to thickening during extended periods under shear, such as occurs during pumping.